QXcbConnection: Could not connect to display, when remotely debugging program with Qt5 - linux

Question

PC: Debian 9.4.0, x64, ip 192.168.1.10
Qt: Qt Creator 4.6.1 Based on Qt5.11.0
BeagleBone Black: Debian 9.3.0, armv7, ip 192.168.1.20
I cross-compiled an GUI application for the BeagleBone Black on my PC and I can execute it successfully on my BeagleBone.
But when I remotely debug the application with F5(Menu: Debug->Start Debugging), I encounter an issue as follow.
QXcbConnection: Could not connect to display
The application crashes in main() at the line:
QApplication a(argc, argv);
The following are the details when compiling and debugging:
debug details:
Checking available ports...
Found 101 free ports.
Starting gdbserver...
Debugging starts
Listening on port 10001
Remote debugging from host 192.168.1.10
Process /home/debian/gdb/armtest3 created; pid = 13981
Unable to find dynamic linker breakpoint function.
GDB will be unable to debug shared library initializers
and track explicitly loaded dynamic code.
Could not load shared library symbols for 25 libraries, e.g. /usr/lib/arm-linux-gnueabihf/libQt5Widgets.so.5.
Use the "info sharedlibrary" command to see the complete listing.
Do you need "set solib-search-path" or "set sysroot"?
-----------------armtest3 start------------
QXcbConnection: Could not connect to display
compile output:
09:31:33: Running steps for project armtest3...
09:31:33: Configuration unchanged, skipping qmake step.
09:31:33: Starting: "/usr/bin/make"
make: Nothing to be done for 'first'.
09:31:33: The process "/usr/bin/make" exited normally.
09:31:33: The remote file system has 218 megabytes of free space, going ahead.
09:31:33: Deploy step finished.
09:31:33: Trying to kill "/home/debian/gdb/armtest3" on remote device...
09:31:37: Remote application killed.
09:31:37: Deploy step finished.
09:31:37: No deployment action necessary. Skipping.
09:31:37: Deploy step finished.
09:31:37: Elapsed time: 00:04.
The following are my BeagleBone Kit：
BeagleBone Kit configuration
main.c
main.c
#include "mainwindow.h"
#include <QApplication>
#include <iostream>
#include <QDateTime>
#include <QDebug>
using namespace std;
int main(int argc, char *argv[])
{
cout << "-----------------armtest3 start------------" << endl;
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
Relevant question: QXcbConnection: Could not connect to display, when trying to debug Linux app with Qt Creator

Answer 1

When you deploy your application and run it from within the X11 system on device itself, you're finding it works because here it has access to the DISPLAY environment variable which (briefly put) tells it where to display itself. This environment variable has been set further up the process tree in your X session.
When you launch a program via the debugger, Qt Creator is connecting to the remote device (via your settings in Tools > Options > Devices), and running the program via ssh. In this context your program no longer knows where to display itself, as obviously it can't display in ssh. It faults on the line you indicated because this is where the XCB subsystem is trying to determine which X-Server to connect to for this purpose.
So to answer your question: you need to manually provide the DISPLAY environment variable when remote debugging.
An easy way to test this is to go to Projects Mode, find the kit you are using to build for the remote device, and select the Run settings. Under this, you should find an Run Environment section. Here you can add a new variable called DISPLAY and set its value to the identifier of the display you are running on (I'm guessing you'll want :0.0, indicating the first available screen on localhost, although you should read about the DISPLAY variable e.g. here or here).
A longer-term, potentially better solution would be to set the same variable in the settings of your Kit (Tools > Options > Build & Run > Kits > Environment). This will then apply for future programs you create using this.